The invention pertains generally to control circuits for the actuation of solenoid devices and is more particularly directed to a driver circuit for controlling the current to the solenoid of an electro-magnetic fuel injector.
For the precise control of the air/fuel ratio of an internal combustion engine electronic fuel injection systems are being widely used today. These systems have an electronic control unit which generates a pulse width signal whose duration is indicative of the quantity of fuel to be input to the engine by sensing the instantaneous engine operating parameters. The pulse width signal controls the opening and closing of a plurality of electro-magnetic fuel injectors having solenoid actuated valves.
During the energized state of the injectors pressurized fuel is metered through the valve by an orifice or nozzle producing a predetermined flow rate. The flow rate multiplied by the actual open time of the injector will determine the quantity of fuel input to the engine. The precision of fuel flow control will be dependent upon matching the actual opening and closing times of the injectors to the duration of the control pulse from the electronic control unit.
Generally, solenoid driver circuits have been used to interface the electronic control unit and the fuel injectors. The solenoid driver circuits attempt to more equally match the mechanical operating characteristics of the valves to the electrical pulse width by regulating the current and voltage levels necessary to open and close the solenoids. The solenoid driver circuits may additionally gate the pulse width to individual groups of injectors.
One such prior art circuit that is known to be advantageous for the operation of this type of solenoid injector includes circuitry for generating a peak current through the coil of the injector solenoid in excess of the amount that is needed to open the valve or "pull in" the armature and then switching to a holding current which is above that at which the armature will close the valve or "drop out." This method insures the rapid opening of the injector by the peak current and the efficient use of power thereafter by the lower holding current which also allows the injector to close more quickly.
However, not all solenoid injectors will operate consistently at their "pull in" current value and therefore the peak value must be set to a point sufficiently high enough to ensure all injectors will be open every time it is reached. This is because it is difficult to predict the exact opening time of an injector by assigning a certain current value as the value may change with age and will not be the same for different injectors. However, the opening of the valve can be predicted to occur sooner at higher battery voltage or later at lower battery voltage because the L/R time constant controlling the rise in current in the coil of the solenoid to the peak value will not change. Thus, to minimize inherent mechanical error, the valve should be opened as quickly as possible with the maximum battery voltage available.
In automotive systems differences in battery voltage may occur under air conditioning loads, cold cranking, and other conditions. The lack of regulation in the battery voltage will cause the injectors to open and close at undeterminable points. This will cause an error which is uncalibrated for in the electronic control unit and which is dependent upon the physical characteristics of the solenoids and variations in the applied source voltage.
At least two previous systems have attempted to overcome this problem. Illustrative of one solution is U.S. Pat. No. 3,725,678 issued to Reddy which is commonly assigned with the present application. The disclosure of Reddy is hereby expressly incorporated herein by reference. Reddy eliminates the opening and closing time differentials due to battery voltage change by closely regulating the voltage applied across the coils of the solenoids during the time the current is building to a peak and during the holding current period.
While advantageously equalizing the solenoid operational characteristics the regulated voltage initially applied to the coils is necessarily less than the full battery voltage available. As previously mentioned the application of the maximum battery voltage available at the opening wll permit the solenoid to operate in the shortest period of time and minimize the inherent mechanical error. This rapid operation is becoming increasingly important as the control pulse widths become shorter and linearity for the injector operation is necessitated into the millisecond operating range.
Another U.S. Pat. No. 4,092,717 issued to Di Nunzio discloses a system for stabilizing the opening time of an electromagnetic injector against variations in vehicle battery voltage. Di Nunzio teaches measuring a battery dependent delay from the termination of the control pulse until an empirically determined "drop out" current is reached and then subtracting that value from the subsequent injection cycle which initiates at an empirically determined "pull in" current.
In this type of system the pulse width indicating an error is not compensated according to deviation presently occurring but by that of a previous pulse width. Counting error in such a system may tend to accumulate rather than cancel. More importantly empirical measurement of two changing values of current for each injector must be made.
Further to more adequately equalize the closing times of the solenoid injectors the prior art has used a Zener diode connected between the coils of the injectors and a reference voltage. The Zener upon the disruption of the current supply to the coils will allow the voltage on the coil to build until it exceeds the breakdown voltage of the diode. Thereafter, the Zener produces a controlled rate of decay or dissipation of the stored energy so that the injectors close at the same rate every time. The time of closure after termination of the control pulse is however a function of the holding current level. The energy stored in the coils at turn off is 1/2Li.sup.2 and thus varies as the square of the holding current. The greater the holding current the longer it will take at the controlled decay rate to reach the drop out current.